Automatic transmissions commonly employ brake bands to engage a desired gearing combination. The brake band wraps around a brake drum. The brake band is restrained on one end. The other end remains free. The band is applied when a force is applied to the free end, displacing it toward the fixed end. This creates tension in the band as it tightens around the drum. As the tension increases, a tangential load between the drum and the band increases. The tangential load decreases the difference in angular velocity between the band and the drum. The drum is fixed relative to the band, and is therefore fixed relative to a transmission case, when the band is fully engaged with the drum.
Two prior art devices which have been employed for restraining brake bands in transmissions are constant diameter pins and ball studs. Both require a boss be provided in the transmission case. The boss is supplied with a constant diameter aperture to retain the pin or stud. Coring the boss in the casting is preferable to not coring because coring reduces both the amount of raw material required and the amount of material removed by machining. Coring also reduces casting porosity by allowing for thinner sections. To minimize transmission case casting complexities associated with the coring of the boss for the pin or stud in the casting, the axis of the aperture in the case for the pin or ball stud is oriented in the same direction as that of the transmission case die pull so the casting core pull is in the same direction.
Beyond the common requirement of being retained within the case, and the preference for having the axis of the pin or stud oriented in the same direction as the die pull, both the constant diameter pins and the ball studs provide advantages and limitations particular to their configuration.
The constant diameter pin is pressed into the transmission case boss aperture. It needs neither a flange to retain the pin nor a specially machined head or angular reaction surface. The pin is effective as a reaction member, but is limited in its ability to be located in a variety of places in the transmission case. The centerline of the pin must pass near the drum's center of rotation. This minimizes the deformation of the brake band anchor bracket produced by contact with the pin and also minimizes axial forces on the pin which would tend to pull the pin into the transmission case.
The requirement that the pin centerline pass near the drum center, combined with the preference for having the centerline of the pin parallel to the direction of draw of the transmission case die, restricts the location of the pin to a number of locations corresponding to the number of dies.
The ball stud, unlike the constant diameter pin, can be located without the need to make its centerline pass near the center of drum rotation. Its centerline is also preferably parallel to the direction of the die pull to minimize casting complexities. Since the contact surface of the ball stud provides a range of tangent surfaces, the anchor bracket will be less likely to slide relative to the pin during a clutch application than if a constant diameter pin were used in a location other than one with its centerline passing near the center of drum rotation. The ball stud must sustain forces trying to pull it into the transmission case. These forces are generated by the application of the brake band when the centerline of the stud is not near the center of rotation of the drum. The ball stud must have a flange on the end, or an alternate method of preventing the stud from being drawn inside.
The ball stud has the disadvantage of providing only point contact with the anchor bracket. This requires precise location of the anchor bracket to ensure that the desired point of tangency on the ball stud is contacted by the brake band anchor bracket. This concern can be reduced by making the anchor bracket sufficiently thick to accommodate the expected variation in the relative positioning of the anchor bracket and the ball stud. A ball stud is also disadvantaged in that a point of contact produces a more localized deformation of the anchor bracket than does a line of contact as found with a constant diameter pin.